1. Field of The Invention
The present invention relates to a dispersion compensation technique for an optical fiber as a transmission path in an optical communication system.
2. Related Background Art
In an optical communication system, an optical fiber is used as a transmission path. A signal light output from an optical transmitter is propagated in the optical fiber to conduct the optical communication. In the past, a laser diode, which emits a laser beam having a center wavelength of 1.3 .mu.m, was used as the optical transmitter and an optical fiber for a 1.3 .mu.m band was installed as the transmission path to conduct the digital transmission (optical communication). Thereafter, it has been found that the wavelength of 1.55 .mu.m exhibits a minimum transmission loss in the glass used as the material of the optical fiber, and it is known to propagate a laser beam having a center wavelength of 1.55 .mu.m through the existing optical fiber in order to attain the communication over a longer distance. However, the laser diode has a specific spectrum width (.DELTA..omega.) in its oscillation wavelength. The propagation speeds in the optical fiber are different between a short wavelength component and a long wavelength component contained in the signal light (dispersion characteristic of the optical fiber). Thus, when the laser beam having the center wavelength of 1.55 .mu.m is propagated through the optical fiber optimized for the 1.3 .mu.m band, the optical signal is distorted so that the transmission distance, the transmission band, and the bit rate are limited.
In order to compensate for the dispersion characteristic of the optical fiber, dispersion compensation techniques disclosed in Japanese Laid-Open Patent Application 62-65529 and Conf. on Optical Fiber Comm. 1992. PD-14, PD-15 have been proposed. An optical fiber having a dispersion characteristic opposite to that of the optical fiber used as the transmission path is inserted to compensate the dispersion characteristic.
On the other hand, the inventors of the present invention have proved that the longer the optical fiber, the more the dispersion compensating fiber and a carrier to noise ratio (CNR) is deteriorated, which impedes good optical communication.